Is it possible to have more fireflies at the same time I use more samples?
First I would recommend checking to see if you are using any small light sources. A sun should be fine but small spotlights and things of that nature can cause fireflies.
Blender Guru has a solution to fireflies. As to why the amount of samples cause more fireflies is strange,  but you are working with a complicated scene.
Blender Guru Fireflies